1. Field of the Invention
The present invention relates generally to magnetic particle test procedures and, more specifically, to a fixture used in a magnetic particle test procedure which produces a uniform magnetic field.
2. Description of the Related Art
In the production of crucial components such as steam turbine blades it is very important to detect manufacturing defects which are not visible to the naked eye. Magnetic particle test procedures have been developed whereby a test sample, such as a turbine blade is pulsed with an electric current to generate a magnetic field therein. Then, a solution containing iron oxide particles is sprayed on the test sample and the particles align themselves in the direction of the magnetic field.
Latent cracks produce localized variations in the magnetic field, and these variations are manifest in the distribution of iron-oxide particles from the sprayed-on solution. When the test sample is illluminated with a black light, the variations in the magnetic field attributable to latent cracks are manifest in observable changes in magnetic field.
While magnetic particle test procedures have been used in the past to test for defects in turbine blades, the shape of the blade has been found to effect the ability to achieve a uniform magnetic field in the test sample. The problem of creating a uniform magnetic field is particularly acute for blades of the type known as triple pin blades in which two or more airfoils shrouded together at the top share a common root portion which has an angled end and two or more root "fingers". In particular, it has been found that the magnetic field does not adequately extend into the fingers and over the sloped surfaces of the end of the root portion. This problem can lead to the failure of the magnetic particle test procedures to detect defects in the root portion of the blade.